Out-of-round pedicle screw

Abstract

A medical bone screw in particular in pedicle screw design includes a screw-and-thread shaft which has a distal shaft portion of a preferably smaller core diameter that transitions into a proximal shaft portion of a preferably larger core diameter, whose proximal end is provided with a screw head. The core cross-sectional shape along the distal shaft portion is formed to be out-of-round, preferably polygonal, and the core cross-sectional shape along the proximal shaft portion is formed to be circular.

The invention claimed is: 
     
       1. A medical bone screw comprising a screw head and a threaded shaft, the threaded shaft having a proximal most end, a distal most end, and a transition between the proximal most end and the distal most end, the threaded shaft comprising:
 a threaded distal shaft portion having a first core and a first axial length, the first axial length beginning adjacent the transition and ending at the distal most end, the first core having a first diameter which is constant along the first axial length of the threaded distal shaft portion; and 
 a threaded proximal shaft portion having a second core and a second axial length, the second axial length beginning adjacent the proximal most end and ending at the transition, the second core having a second diameter which is constant along the second axial length of the threaded proximal shaft portion, the second diameter of the second core being greater than the first diameter of the first core, 
 the first core being formed to be out-of-round along the first axial length and the second core formed to be circular along the second axial length, 
 wherein the first core has a first cross-sectional shape, the first cross-sectional shape being a polygon with a number of corners and equal legs, and 
 the outer diameter of the screw along the first axial length is circular irrespectively of the first cross-sectional shape of the first core. 
 
     
     
       2. The medical bone screw according to  claim 1 , wherein the number of corners changes along the first axial length of the threaded distal shaft portion. 
     
     
       3. The medical bone screw according to  claim 1 , wherein the threaded distal shaft portion comprises a plurality of threads, and wherein the corners on the first core between respectively longitudinally adjacent screw threads are arranged one behind the other at the same angular position along the threaded distal shaft portion. 
     
     
       4. The medical bone screw according to  claim 1 , wherein the threaded distal shaft portion comprises a plurality of threads, and wherein the corners on the first core between respectively longitudinally adjacent screw threads are arranged so as to be angularly staggered by an angular displacement with respect to each other. 
     
     
       5. The medical bone screw according to  claim 4 , wherein a direction and/or an amount of the angular displacement changes continuously and/or abruptly along the first axial length of the threaded distal shaft portion. 
     
     
       6. The medical bone screw according to  claim 1 , wherein the threaded shaft is manufactured by a thread whirling process utilizing a thread whirling tool at least in a zone of the threaded distal shaft portion. 
     
     
       7. The medical bone screw according to  claim 1 , wherein the threaded proximal shaft section and the threaded distal shaft section comprise a plurality of screw flanks, the screw flanks comprising outer edges each having a diameter, the outer edges of the screw flanks, even along the threaded distal shaft portion, form a continuous spiral, the diameters of the outer edges following the core diameter. 
     
     
       8. The medical bone screw according to  claim 7 , wherein the screw flanks in the distal shaft portion are shaped so as to have no kinks or sharp edges. 
     
     
       9. The medical bone screw according to  claim 1 , wherein a sleeve-shaped tulip for receiving a longitudinal rail is pivotally coupled to the screw head. 
     
     
       10. A medical bone screw comprising a screw head and a threaded shaft, the threaded shaft having a proximal most end, a distal most end, and a transition between the proximal most end and the distal most end, the threaded shaft comprising:
 a threaded distal shaft portion having a first core and a first axial length, the first axial length beginning adjacent the transition and ending at the distal most end, the first core having a first diameter which is constant along the first axial length of the threaded distal shaft portion; and 
 a threaded proximal shaft portion having a second core and a second axial length, the second axial length beginning adjacent the proximal most end and ending at the transition, the second core having a second diameter which is constant along the second axial length of the threaded proximal shaft portion, the second diameter of the second core being greater than the first diameter of the first core, 
 the first core being formed to be out-of-round along the first axial length and the second core formed to be circular along the second axial length, 
 wherein the first core has a first cross-sectional shape, the first cross-sectional shape being a polygon with a number of corners, and 
 wherein the number of corners changes along the first axial length of the threaded distal shaft portion. 
 
     
     
       11. A medical bone screw comprising a screw head and a threaded shaft, the threaded shaft having a proximal most end, a distal most end, and a transition between the proximal most end and the distal most end, the threaded shaft comprising:
 a threaded distal shaft portion having a first core and a first axial length, the first axial length beginning adjacent the transition and ending at the distal most end, the first core having a first diameter which is constant along the first axial length of the threaded distal shaft portion; and 
 a threaded proximal shaft portion having a second core and a second axial length, the second axial length beginning adjacent the proximal most end and ending at the transition, the second core having a second diameter which is constant along the second axial length of the threaded proximal shaft portion, the second diameter of the second core being greater than the first diameter of the first core, 
 the first core being formed to be out-of-round along the first axial length and the second core formed to be circular along the second axial length, 
 wherein the first core has a first cross-sectional shape, the first cross-sectional shape being a polygon with a number of corners, and 
 wherein the threaded distal shaft portion comprises a plurality of threads, and wherein the corners on the first core between respectively longitudinally adjacent screw threads are arranged so as to be angularly staggered by an angular displacement with respect to each other. 
 
     
     
       12. The medical bone screw according to  claim 11 , wherein a direction and/or an amount of the angular displacement changes continuously and/or abruptly along the first axial length of the threaded distal shaft portion.